Both diaryl ketones and alkyl aryl ketones long have been staples of commerce with a wide variety of uses, both in end applications and as valuable intermediates in numerous chemical syntheses. Using benzophenone as an example which is an important subject of this application, the ketone and its substitutes have found use as photoinitiators for ultraviolet-curable printing inks and coatings as well as an ultraviolet stabilizer in plastics. It is employed in the perfume industry as a fixative and as a fragrance with a flowery note, and as an intermediate is used both in the pharmaceutical and agricultural chemical industries.
Benzophenone is usually produced by atmospheric oxidation of diphenyl methane in the presence of metal catalysts. Commercial production also can be effected by Friedel-Crafts acylation of benzene with benzyl chloride or of benzene with phosgene.
Friedel-Crafts acylation of aromatics to afford both diaryl and aryl alkyl ketones is a time honored method of preparation affording a broad chemical spectrum of ketones. Acid halides are the most prevalent acylating agents, most likely because of their high reactivity in the acylation of aromatic compounds. One unfortunate adverse use of acid halides as the acylating agent is the formation of the corresponding hydrogen halide as a reaction product. Since acid chlorides are by far the most commonly employed acid halide, hydrogen chloride is the hydrogen halide normally formed and, as is well known, is a highly acidic, corrosive material whether as a gas or in solution. Formation of such a corrosive acid is a major detriment in Friedel-Crafts acylation and is desirably to be avoided wherever this is feasible.
Even though a broad spectrum of Friedel-Crafts catalysts are known, aluminum chloride is the archetype because of its activity and near universal applicability as well as for its relative availability and low cost. However, aluminum chloride is not without its own problems. For example, it is extremely sensitive to moisture which causes formation of hydrogen chloride. Although it can be used as a solid acidic catalyst in a continuous process, its flow characteristics are far from desirable. Not the least of its disadvantages is the difficult problem associated with disposal of spent catalyst; because it is corrosive and difficult to handle, aluminum chloride poses somewhat of an environmental hazard. That all these difficulties can be dealt with is clear from the fact it has been used as a Friedel-Crafts catalyst for many decades.
In the course of investigating preparative methods for benzophenone, the various oxidative methods quickly proved disadvantageous. Just as quickly it became clear that Friedel-Crafts acylation would be most desirable so long as certain attributes could be incorporated into a procedure. In particular, it became apparent that acid halides were undesirable acylating agents and that aluminum chloride, and similar materials used as Friedel-Crafts catalyst, were not acceptable. Although acid anhydrides long have been used as an acylating agent, they often are disfavored both because of their low reactivity relative to acid halides and because only one of the acid residues is incorporated as a carbonyl group into the ketone, with the other acid residue merely regenerating the carboxylic acid. Since the acid anhydride generally is made from the acid, it can be seen that the use of acid anhydrides is in some sense an inefficient utilization of the acid from which the anhydride is prepared.
Although a solid catalyst was desirable in order that a continuous process for the preparation of diaryl and aryl alkyl ketones could be designed, it was equally apparent that aluminum chloride, and materials similar to it, simply were not acceptable catalysts. Consequently it was necessary to find solid acidic catalysts that would function well in the acylation of aromatic compounds and which could be used in a continuous process for ketone preparation.
Some solid acidic catalysts already have been noted as acceptable substitutes for aluminum chloride, at least in particular acylation reactions. For example, Kawamata et al. U.S. Pat. No. 4,459,234 teach the preparation of anthraquinone in the vapor phase reaction of phthalic anhydride with benzene in the presence of a titanium or tin oxide catalyst. At high mole ratios of benzene to phthalic acid and at temperatures on the order of 500.degree. C. the patentees observed over 90% conversion of phthalic acid with a selectivity to anthraquinone on the order of 80%. Somewhat analogous is JP76001710 which describes the preparation of anthraquinone from phthalic anhydride and benzene in the gas phase using group I metal aluminosilicates of the faujasite type as a catalyst. GB1499276 describes the vapor phase acylation of, for example, benzene and xylene at 250.degree.-500.degree. C. in the presence of certain silica-alumina catalysts.
What we have observed, and what forms the basis for the invention as claimed herein, is the reaction of aromatics, and especially aromatic hydrocarbons, with acid anhydrides, and especially aromatic carboxylic acid anhydrides, in the liquid phase at temperatures generally under 350.degree. C. using a variety of solid acidic catalysts, including certain clays and sulfated metal oxides having extremely high acidity. Recently Cornelis et al., Catalysis Letters, 6, 103-10 (1990), have reported that various metal-impregnated and metal cation-exchanged clays are effective catalysts in liquid phase Friedel-Crafts acylation at temperatures on the order of 150.degree. C. Although most acylations were performed using acid chlorides, the particularly reactive substrate anisole was acylated quantitatively at 160.degree. C. using benzoic anhydride as the acylating agent. Kaolin, montmorillonite, and an otherwise unidentified "Japanese acidic" clay were the only ones reported.